Fluid catalytic cracking (FCC) of petroleum fractions is a well-established refinery operation. In FCC, heavy hydrocarbon fractions (greater than about 20 to about 30 carbon atoms in length) are chemically broken down into lighter hydrocarbon fractions (less than about 12 to about 15 carbons in length), such as gasoline. The FCC unit usually comprises a reactor section connected to a regenerator section by standpipes. The catalyst itself is a finely divided solid and behaves like a fluid in the reactor, regenerator, and connecting standpipes, hence the designation "fluid" catalyst.
In the operation of the FCC process, fresh hydrocarbon feed, which may be preheated, is mixed with catalyst and undergoes cracking within the catalytic conversion zone of the reactor section. The catalytic conversion zone, in modern FCC units, is primarily located in the riser of the reactor section. For catalytic cracking to occur, the hydrocarbon feed (e.g., oil), must be vaporized to permit the hydrocarbon feed to diffuse into the pores of the catalyst (generally a zeolite) to the cracking sites. The catalytic cracking reaction results in coke being deposited onto the catalyst to form "coked" or "Spent" catalyst. Products exit the reactor in the vapor phase and pass to at least one main fractionator or distillation column for separation into desired fractions. The spent catalyst passes continuously from the reactor to the regenerator via a spent catalyst standpipe. In the regenerator, the coke is converted in an exothermic reaction to gaseous products by contact with an oxygen-containing gas. The flue gas passes out of the regenerator through various heat recovery means, and the hot regenerated catalyst is recirculated to the reactor via a return catalyst standpipe where it is again picked up by fresh hydrocarbon feed. Typically, heat released in the regenerator is carried to the reactor by the hot regenerated catalyst to supply heat for the endothermic cracking reactions. Typical fluid catalyst cracking systems are disclosed in U.S. Pat. Nos. 3,206,393 to Pohlenz and 3,261,777 to Iscol, et al., which are hereby incorporated by reference in their entireties.
Nozzles are used to inject the hydrocarbon feed, typically in the form of a liquid spray, into the catalytic conversion zone of the riser. To form a spray, typically, the hydrocarbon feed is combined with a dispersion medium, such as steam, to form a dispersed hydrocarbon stream. The one or more nozzles used to inject the dispersed hydrocarbon stream into the catalytic conversion zone may be axially or radially disposed.
With axial nozzles, coverage is achieved by using one or more nozzles that extend into the riser section of an FCC unit and terminate at a set of points within the riser cross-sectional area. The axial nozzles are preferably oriented almost or perfectly vertical (preferably within about 10.degree. of the vertical axis of the riser) to create a flow of hydrocarbon feed that is preferably parallel to the upflowing catalyst. With radial nozzles, coverage is achieved by using a plurality of nozzles that are mounted around the perimeter of the riser wall. Preferably, the radial nozzles extend minimally into the riser itself. This orientation of the nozzles creates a flow of hydrocarbon feed that crosses with the upward flow of catalyst. Radial nozzles are preferably oriented with respect to the vertical axis of the riser at any angle from about 10.degree. pointing upward to about 90.degree. horizontal). To provide optimal catalytic cracking conditions, nozzles in either orientation must collectively spray the dispersed hydrocarbon stream in a pattern that expands to cover the entire cross-sectional area of the riser through which the cracking catalyst is flowing. Improved coverage provides better catalyst-hydrocarbon feed mixing that enhances catalytic cracking reactions and minimizes thermal cracking reactions. Thermal cracking reactions produce undesirable products such as methane and ethane that lead to decreased yields of more valuable FCC products.
In addition to full spray coverage, the nozzles should produce fine hydrocarbon feed droplets that are preferably comparable to the size of individual catalyst particles. Preferably the hydrocarbon feed droplets have a Sauter mean diameter (i.e., the diameter of a sphere having the same volume to surface area ratio as the measured droplets) of less than 100 microns (.mu.m). As droplet size decreases, the ratio of hydrocarbon feed drop surface area to volume increases, which accelerates heat transfer from the catalyst to the hydrocarbon feed and shortens hydrocarbon feed vaporization time. Quicker vaporization improves yield of catalytic cracking reaction products since the hydrocarbon feed as a vapor is able to diffuse into the pores of the catalyst. Conversely, any delay in hydrocarbon feed vaporization, and/or mixing of the hydrocarbon feed and catalyst, leads to increased yields of thermal cracking products and coke.
The majority of FCC nozzles in use today, whether radial or axial, employ high velocities (e.g., about greater than 300 feet per second) of the dispersed hydrocarbon stream to shear the hydrocarbon feed into small droplet for injecting the droplets into the catalytic conversion zone. However, high velocities can have undesirable effects. For example, temperature profiles in commercial units having radial injection sometimes show significantly lower temperatures along the center vertical axis in the catalytic conversion zone of the riser. This temperature profile indicates that the catalyst and feed drops are not being uniformly mixed across the cross-sectional area of the riser. Particularly, the cooler liquid feed droplets are travelling to the center of the riser without exchanging a significant amount of momentum and heat with the catalyst. Therefore, a nozzle is needed that can provide adequate shear at a dispersed hydrocarbon feed velocity which leads to adequate mixing of the hydrocarbon feed droplets and catalyst.
Additionally, many FCC radial nozzles in use today have poor coverage. This problem can be seen when a nozzle is installed which produces smaller droplets, but no increase in yield is observed due to poor contacting of the droplets and catalyst.
U.S. Pat. No. 4,601,814 to Mauleon et al., (hereinafter "Mauleon") discloses a high-velocity radially oriented nozzle for atomizing residual oils in a catalytic cracking process. In one embodiment, the nozzle end is a single horizontal restricted slot opening that creates a fan-shaped spray pattern. Mauleon also discloses that the nozzle end may be two parallel slots or two slots 90.degree. to one another. The discharge velocity of the atomized feed oil is high, exceeding 300 feet per second and more preferably 500 feet per second. However, for the reasons previously stated, a nozzle requiring high velocities may be ineffective.
Thus it is desirable to develop a nozzle and nozzle assembly that produces a spray of fine hydrocarbon liquid droplets that covers the entire cross section of the catalytic cracking zone without requiring high nozzle velocities.